Expand docs on Windows data and binary directories

[SpiralBrad]

Describe the bug

The Installation section of the docs states that "restic will be downloaded and installed in the data directory on first run." - Upon running backrest-tray.exe and attempting to "Open WebUI" - the URL doesn't work. - Checking the logs I see the following:

{"level":"info","ts":1719676879.462121,"msg":"installing restic C:\\Program Files/backrest/restic-0.16.4.exe to 0.16.4..."}
{"level":"fatal","ts":1719676879.4626205,"msg":"error finding or installing restic: install restic: create restic install directory C:\\Program Files/backrest: mkdir C:\\Program Files/backrest: Access is denied."}

(Interesting path slashes you have there... that's fixable...) But yes, this happened because I did not run it as Administrator, nor have C:\Program Files\backrest in place with User write permissions. The kicker: this differs from the Installation docs which state that it will be "downloaded and installed in the data directory on first run"...

What even is "the data directory"? My assumption would be one of two things: the target location for the backup, or the directory where backrest resides. ...Not Windows' Program Files.

To Reproduce

1. Download backrest on a 'clean' Windows system
2. Run backrest-windows-tray.exe without UAC elevation
3. Notice WebUI does not work.

Expected behavior

1. Download backrest on a 'clean' Windows system
2. Run backrest-windows-tray.exe without UAC elevation
3. restic gets downloaded to the current folder/same directory as backrest
4. WebUI works as expected

Platform Info Windows 11 x64 Backrest 1.2.0